Interpretive Summary: Biological control is an important component of integrated management systems for flies associated with livestock, poultry and horses. Parasitic wasps are among the most important biocontrol agents for flies, and there are several commercial insectaries that provide these wasps to farmers. There are about a dozen native species of wasps that are available for commercial production, but it remains unclear which species are most effective in different situations. Moreover, most of this research has been conducted on poultry and cattle operations. Horse-owners are among the important customers of commercial insectaries, yet little is known about the effectiveness of various species in habitats associated with horse farms. This study, conducted by researchers at the University of Florida and USDA-ARS’s Center for Medical, Agricultural and Veterinary Entomology (CMAVE, Gainesville, FL) was conducted to compare the relative effectiveness of three commonly-sold species of parasitic wasps to kill stable fly pupae in soiled horse bedding. The three species (Spalangia cameroni, S. endius, and M. raptorellus) were equally effective at finding and killing fly pupae when the pupae were placed in a manner that made them easy to find. However, when the pupae were sparsely distributed in soiled horse bedding in manner more typical of field conditions the two Spalangia species were 50 times more effective at finding them than were M. raptorellus. The results suggest that parasitoid products for fly control on horse farms should be composed of a high proportion of Spalangia spp and few if any Muscidifurax spp.

Technical Abstract:
Laboratory evaluations assessing the ability of Spalangia cameroni Perkins, Spalangia endius Walker, and Muscidifurax raptorellus Kogan and Legner to locate and attack stable fly hosts in a field-collected fly-breeding substrate suggest that Spalangia spp. are more suited to successfully locate and attack hosts in habitats created by equine husbandry. On average, parasitism rates of freely accessible stable fly pupae were not significantly different between parasitoid species. However, when post-feeding, 3rd stage stable fly larvae were allowed to disperse and pupate freely within an arena containing field-collected soiled horse bedding, parasitism rates by both Spalangia spp. were approximately 50-fold that of M. raptorellus. Additional intraspecies analysis revealed that parasitism rates by both S. cameroni and S. endius were not significantly different when pupae were freely accessible or within bedding, whereas significantly fewer pupae were attacked by M. raptorellus in the arenas having pupae distributed within bedding. Therefore, commercially available parasitoid mixtures containing Muscidifurax spp. may be ineffective if used as a control measure at Florida equine facilities.